The present invention is related to accessory systems for gas turbine engines, and more particularly to fluid pump systems for gas turbine engines.
Gas turbine engines operate by passing a volume of gas through a series of compressors and turbines in order to produce rotational shaft power. The shaft power is used to turn a turbine for driving a compressor to generate compressed air. The compressed air is mixed with a fuel and combusted within a combustor to generate high energy gases. The high energy gases sustain rotation of the turbines and produce thrust. Additionally, the shaft power can be used to rotate a fan to produce additional thrust.
In aircraft engines, the shaft power is typically used to provide power to accessory systems needed to operate the engine and the aircraft. For example, accessory gear boxes provide input power to generators for providing electrical power to the engine, as well as input power to pumps for systems that provide fuel and lubricant to the engine. Such gear boxes are typically driven through a tower shaft connected to a turbine shaft in the engine. As such, the accessory gear box operates at speeds directly corresponding to operational speed of the engine. Thus, the displacements of fuel pumps and lubrication pumps driven by the accessory gear box must be sized to meet required output according to engine speed parameters, rather than fuel and lubrication requirements. As such the displacements are often oversized for their expected operating ranges. The problem is compounded as typical fuel and lubrication systems often require ancillary scavenge pumps and boost pumps, respectively, in addition to the primary fuel pump and lubrication pump. Furthermore, tower shafts and gear boxes required for pump operation extend from engines in such a manner so as to require large engine nacelles that increase the frontal area of some airplanes, increasing drag and reducing fuel efficiency.
In order to reduce problems associated with engine nacelle sizing, the pump gearbox can be eliminated, which also uncouples the pump systems from the turbine shaft and mitigates pump displacement design limitations. The pump systems can be powered with a separate electric drive motor for each pump. The desired amount of fluid flow from each pump can then be provided independent of engine speed. Furthermore, each pump and motor can be incorporated into the engine nacelle without unduly increasing the frontal area of the engine and hence the airplane. Such a system, however, adds significant weight and cost to the engine. Alternatively, each pump could be driven by a single electric motor with the addition of metering valves and regulators. Such a system enables control over the motor speed independent of engine speed, but suffers from the same sizing and displacement problems as gearbox-driven pumps; each pump is sized based on the single speed at which the electric motor operates rather than individual flow requirements. There is, therefore, a need for lightweight and compact pump systems that allow for greater control of flow from individual pumps.